You know, one thing that I forgot to mention, but it's vitally important to remember: if you find out you didn't test something, you might want to ask yourself if you have dead code. In your example, do you ever call that code in scalar context? (well, void context would also exercise that condition, but we'll ignore that) If not, you have dead code. If you do call it in scalar context, why are you overloading the return value? If you don't need to, perhaps this is an opportunity to simplify your code and thereby reduce the chances of bugs.

Personally, I think something like that is too trivial to test -- until it breaks. If that's the only thing that Devel::Cover says you've missed, go ahead and toss the test in there if you like. Otherwise, concentrate on the "big ticket items" and only test that if it breaks (or if you feel like it. Some people are happy to test everything).